Transplant-associated arteriopathy (TAA) constitutes the major cause of graft failure and death in recipients who survive more than one year after cardiac transplantation, despite advances in immunosuppression. One of the early events leading to TAA is infiltration and activation of peri-adventitial inflammatory cells, including T cells and macrophages, which is followed by the formation of a diffuse, concentric neointima in which smooth muscle cells and leukocytes accumulate. Both T cell effectors and T regulatory cells play key roles in regulating the balance of allograft inflammation. Interestingly, age-related diminution of T cell responsiveness to mitogen signals have been linked to improved allograft survival. Indeed, epidemiological studies indicate that older recipients have reduced allograft rejection rates than younger recipients. As such, identification of factors that reduce activation of T cells could potentially impact both acute and chronic cardiac allograft rejection. Kruppel-like factors are transcriptional regulators of cell growth, differentiation, and activation. We identified a member of this family, KLF10, whose expression is highly expressed in T cells and increases with age. We identified that the TGF-b1-responsive factor, KLF10, regulates key aspects of both T cell effector and T regulatory cell function. Our preliminary studies uncover a novel role for KLF10 in age-related diminution of T cell activation in TAA. Major histocompatibility complex class II (MHC II)-mismatched cardiac allografts showed similar TAA and survival when transplanted into young recipients of either wild-type (WT) or KLF10-/- mice. In contrast, allografts in older KLF10-/- recipients developed accelerated TAA with increased accumulation of peri- adventitial T cells and macrophages, increased levels of pro-inflammatory markers (e.g. IFN-?, TNF-a, IL-6, and MCP-1), and reduced survival. Older KLF10-/- CD4+ T cells exhibited enhanced Th1 and decreased T regulatory cell (Treg) function with reduced expression of CTLA-4, an immunosuppressive molecule implicated in T cell responsiveness with aging. Finally, cardiac allografts transplanted into younger KLF10-/- hosts receiving older KLF10-/- T cells (vs. younger KLF10-/- T cells) showed increased TAA. Mechanistic studies demonstrate that KLF10 is targeted by a microRNA, miR-340, that is inversely expressed with KLF10 with increasing age to regulate TGF-b1/CTLA-4 signaling. These observations provide the foundation for our central hypothesis that KLF10 serves as a critical regulator of T cell responsiveness with aging and TAA. In Aim1, we will explore the upstream mechanisms regulating KLF10 expression in old and young T cell effectors. In Aim2, we will determine the molecular basis for impaired Th1 and Treg cell function in older CD4-KLF10-deficient mice critical for TAA. In Aim3, we will explore the effect of altered KLF10 expression on T cell responsiveness and experimental TAA using young and old, T cell-specific KLF10-KO mice and anti-miR-340 therapeutics. The results of these studies will provide considerable insights regarding KLF10 function in T cell biology, aging, and may serve as the basis for novel therapeutic strategies to modulate T cell responsiveness and TAA.